A study of uranium phosvitin complexation using ATR FT-IR and EXAFS spectroscopy

Phosvitin is a highly water soluble 34 kDa protein containing 10 % (w/w) phosphate groups. It serves as an ideal model for the investigation of the interaction of U(VI) with phosphorylated proteins by attenuated total reflection Fourier-transform infrared (ATR FT-IR) spectroscopy and EXAFS spectroscopy.
In our study, phosvitin and its complex with U(VI) were first studied in aqueous solution at various pH values by ATR FT-IR. In order to understand the inter-atomic coordination of the U(VI) to the functional groups of the protein, in particular to phosphate groups, the samples containing U(VI) phosvitin complex with different U(VI) concentrations were additionally investigated by EXAFS spectroscopy.
Different protonated states of phosphate groups on the protein have been first identified by infrared spectra of phosvitin in solution at different pH. Comparing the spectra with those of inorganic phosphate solutions, it is obvious that at pH 4 the phosphate groups appear as dihydrogen phosphate, and at pH 8 as hydrogen phosphate.
Upon the adsorption of uranium onto phosvitin the protein is irreversible denatured. Infrared spectra of the protein, which were denatured thermally in the absence of U(VI) or denatured by other bivalent metal ions, recorded at pH 4 demonstrate that deprotonation occurs during the protein denaturation. Among those spectra, the U(VI) phosvitin complexes with lower U(VI) concentrations (10−4 M, and 10−5 M) show a unique peak at or even lower than 918 cm−1, while this peak appears at 925 cm−1 for the complex with higher U(VI) concentration (10−3 M). As the characteristic νas(UO22+) mode representing a U(VI) complexation to carboxylic groups appears at 923 cm−1 on the IR spectra, the discovered unique peak indicates coordination of the U(VI) predominately to phosphate groups of the phosvitin.
The EXAFS spectra of the U(VI) phosvitin complex are in good agreement to the spectra of other organic U(VI) phosphate complexes confirming the results from IR spectroscopy. The formation of a U(VI) complex, which is similar to the so-called “Feldman complex” is discussed.